Now that winter is in full swing, you may have noticed frost on your car windows each morning. If blasting your car’s HVAC climate controls doesn’t clear the ice and interior fog, there may be a problem with your rear defroster or electrical system.
How Does The Rear Defroster Work?
Defrosting warms your car’s windshield and rear window to melt exterior snow, ice, and frost. The rear window features thin heating elements that warm the glass. This warmth improves rear visibility by melting ice and snow. Fog removal is handled by the HVAC system, since fog forms inside the cabin.
Climate Controls
The climate controls on your dashboard turn on the air conditioning, heating, and rear defroster functions. Older model cars may have knobs, buttons, or dials with fan settings to control the flow of cold or warm air through the vents. Newer (and high-end) cars may have digital thermostats and electronic sensors that control cabin temperature and airflow. To engage the rear defroster on most model vehicles, press the button (or switch) with an icon of a rectangle with wavy lines or a snowflake. Once the rear defroster button is on, it may light up and run for a specific time – usually about 10 minutes – before turning off.
HVAC
Your car’s HVAC (or A/C and Heating system) dehumidifies the air, helping clear fogged windows inside the cabin. The HVAC system defrosts the front windshield by blowing warm air directly onto the glass through vents in the dashboard. Since airflow is concentrated at the front of the vehicle, the rear window defrosts using electrical elements.
Reasons Why Rear Defroster Won’t Work
The rear defroster relies on electrical components instead of the HVAC to melt snow, ice, and frost from the glass. The dashboard climate controls contain the rear defroster dial (or button) to turn on the electrical components to heat the window. If any of the many electrical components are compromised, your rear defroster may not work.
Blown Fuse
An electrical short or blown fuse can disrupt the rear defroster’s function, causing it to stop working. Usually, your owner’s manual will contain a diagram of the fuse box. Find which fuse controls the rear defroster and check whether it requires replacement.
Broken Gridline
Any scratched or broken gridlines in the back window will not effectively conduct electricity. Breaks or gaps in the gridlines could lead to uneven or incomplete defogging of the rear window.
Damaged Wiring, Relay Switch or Terminal
The relay in your vehicle’s electrical system controls power distribution to the back window. If the relay fails, the rear defroster will not receive power. There are also small terminals in the rear window that the defroster uses to connect to the grid in the glass. Loose or corroded terminals can be problematic, as well as damaged wiring.
Dirty Window
Excessive road grime, bird droppings, tree sap, or other debris on your rear window can reduce the effectiveness of your defroster. You can gently clean your rear window with glass cleaner, both inside and out, to help reduce uneven or inconsistent defrosting.
Window Tinting
Aftermarket window tinting or decorative window wraps can interfere with the gridline’s conductivity. If you suspect this may be an issue, speak with your service technician to see if they can offer a solution or repair option.
HVAC & Rear Defroster Repair in Kirkland, WA
Fisher’s Auto Service in Kirkland, WA, provides HVAC and rear defroster inspection, maintenance, and repairs for all model SUVs, trucks, and cars. Our ASE-certified service technicians are true experts in our field.
Make An Appointment
If you need assistance getting your car repaired, give us a call at (425) 823-4441 or visit our website to make an appointment. We look forward to hearing from you!